1. Field
Exemplary embodiments relate to a semiconductor memory device, and more particularly, relate to a non-volatile memory device, a memory system, and a read method thereof.
2. Description of the Related Art
Semiconductor memory devices may be generally classified into volatile memory devices and non-volatile memory devices. Volatile memory devices may lose stored contents when a supply of power is removed therefrom, while non-volatile memory devices may retain stored contents even when a supply of power is removed therefrom. Non-volatile memory devices may be formed of various types of memory cell transistors. Non-volatile memory devices may include flash memory device, ferroelectric random access memory (FRAM) devices, magnetic RAMs (MRAMs), phase change RAMs (PRAMs), or the like.
As one type of non-volatile memory device, flash memory devices, may be divided into NOR type flash memory devices and NAND flash memory devices according to connection relationships of memory cells and bit lines. NOR flash memory devices may have a structure wherein two or more memory cell transistors are connected to one bit line in parallel. Accordingly, NOR flash memory devices may have an excellent random access time characteristic. On the other hand, NAND flash memory devices may have a structure wherein two or more memory cells are connected to one bit line in series. This structure is called a cell string structure. One cell string may require one bit line contact. Accordingly, NAND flash memory devices may have an excellent device integration characteristic.
Memory cells of a flash memory device may be determined to be ON cells or OFF cells according to a threshold voltage distribution. An ON cell may represent an erased cell, and an OFF cell may represent a programmed cell. A programmed memory cell may have one threshold voltage belonging to one of a plurality of threshold voltage distributions each corresponding to N program states (or, programmed data values) (N being an integer of 1 or more).
At programming, a coupling effect may occur between selected memory cell and adjacent memory cell. The coupling effect may make a threshold voltage distribution corresponding to the selected memory cell become wide and a margin between adjacent threshold voltage distributions become narrow. Such a coupling effect may be called “electric field coupling” or “F-poly coupling”. If a variation of the threshold voltage distribution corresponding to the selected memory cell and a reduction of the margin between adjacent threshold voltage distributions are caused due to the coupling effect, it is impossible to reliably read data from memory cells. This problem may increase in proportion to an increase in the number of data bits being stored per cell.